This is part of a monthly series on growing potatoes, a dietary staple.
PART ONE: Planting potatoes (April)
PART TWO: Growing potatoes (May)
PART THREE: Potato pests and diseases (June)
PART FOUR: Harvesting potatoes (July)
PART FIVE: Storing potatoes (August)
PART SIX: Planning to grow potatoes again (September)
I have a whole chapter about potatoes in my book, Sustainable Market Farming, and another on root cellars (including construction), where much of this information can be found.
This year’s new Victory Gardeners now need to learn how to store your harvest, so it can supply your household for as long a s possible. As more commercial growers aim to produce local food sustainably year-round, the storage of vegetables for sale over the winter becomes important. Understanding the needs of different crops can help reduce your electricity bill and carbon footprint, and maximize the amount of produce you can store for later sale. Only critical crops need refrigeration. Potatoes should not be refrigerated. Many others may be stored without electricity, perhaps in buildings that serve other uses at the height of the growing season.
A 1978 publication from Washington State University Extension, Storing Vegetables and Fruits at Home, is a good introduction to alternatives to refrigerated storage, using pits, clamps and root cellars. There is also good information in USDA Agriculture Handbook 66. 2016 revision. Many growers are still using the 1986 version, but it’s worth checking newer recommendations and additional advice. UMass Extension has a good site on post-harvest and storage resources. Nancy and Mike Bubel’s book Root Cellaring has a wealth of information, including how to build a root cellar.
Don’t expect a one-shed-fits-all solution to crop storage. I identify five different sets of storage requirements, for different storage crops. This one is specific to potatoes, which don’t want colder conditions: Cool and moist: 40°F–50°F (5°C–10°C), 85%–90% humidity. With a good in-ground root cellar, potatoes can be stored for 5-8 months, but other options can also work. A max-min thermometer will help you keep the storage space in the right range.
Only store sound potatoes. Garbage in, garbage out. Damaged and poor quality vegetables will not store well. Always handle all crops for long-term storage gently, to avoid bruising. For long-term storage, make sure crops are fully mature but not over-mature when you harvest. Potatoes need firm skins that don’t rub off when you rub with a thumb. This is different from some crops, such as beets and sweet potatoes, that don’t have a “ripe” stage, but are ready when they reach the size you like. Very small vegetables don’t store well. Expect that a small percentage of your crops will go bad in storage — it’s not a sign of failure, just a reminder that life has limitations.
Cure, then store
Potatoes are one of those vegetables that need to cure before storage in conditions that are different from those needed for storage. Curing allows skins to harden and some of the starches to convert to sugars. See the post on Harvesting Potatoes in July. Potatoes need curing in moist air (90% humidity) for one to two weeks at 60°F–75°F (15°C–24°C). You may be surprised at how warm this is. Wounds in the skin will not heal below 50°F (10°C).
We sort our potatoes after two weeks of curing and find this usually reduces the chance of rot so that we don’t need to sort again. With potatoes, the rate of deterioration drops right down after a few weeks. Remember to keep white potatoes in the dark while curing as well as during storage.
Preparation for storage
Plan your storage sites, buy a thermometer for each site, and gather suitable containers. Clean and prepare your storage space before going out to do a big harvest. Wood crates are good for nostalgia and agritourism, but plastic is kinder on aging backs and less likely to harbor diseases. Containers should rest on shelves, pallets or blocks of some kind, and not be set on bare concrete floors. This helps improve ventilation and reduce condensation.
For traditional storage without refrigeration, potatoes (and most other root crops) store best unwashed (less wrinkling), though this can make them harder to clean later. If you might not be able to keep temperatures low enough, choose stackable crates rather than closed bags. When you have choice in the matter, try to harvest potatoes from relatively dry soil, so they are less likely to grow mold. The packing of your containers should allow for airflow, but you don’t want the produce to shrivel up, so be observant. Sometimes night ventilation offers cooler, drier air than you can get in the daytime. Keeping root cellar temperatures within a narrow range takes human intervention, or sophisticated thermostats and vents. If needed, electric fans can be used to force air through a building.
Ethylene is an odorless, colorless gas, generally associated with ripening, sprouting and rotting. Chilling, wounding and pathogen attack can all induce ethylene formation in damaged crops.
Some crops, including most cut greens, are not very sensitive to ethylene and so can be stored in the same space as ethylene-producing crops. Potatoes are very sensitive to ethylene and will sprout in a high-ethylene environment.
Some crops, such as ripening fruits, produce ethylene gas while in storage. Don’t be tempted to set that bargain box of very ripe bananas you bought on the way home near anything you don’t want to sprout or ripen further. Propane heaters and combustion engines produce ethylene. Be careful if using your garage to store potatoes.
Basement storage rooms and root cellars
Traditional root cellars are made by excavating a large hole near the house, lining it with block or stone-work walls, casting a well-supported and well-insulated concrete roof, then covering the top with a big mound of soil. The doorway may have bulkhead doors or an entry way with additional doors. The more modern version is to construct an insulated cellar in the basement of a building such as a CSA distribution barn or your house. See the Bubels’ book, or the Washington State publication for drawings and instructions on making these. Provide wide doorways with ground-level access if possible (roll that garden cart right in!). Good lighting and drainage are important, so you can see if everything is storing well, or hose the shelves and floor down if it isn’t. Mouse-proofing is worth considering upfront. Our 10’ x 11.5’ (3 x 3.5 m) cellar will hold 360 crates with an ample central path. That’s 10,800 pounds (4900 kg), or around 5 tons (tonnes).
Root Cellar Ecosystem
Store potatoes in a moist, completely dark cellar, ideally at 40°F (5°C), up to 50°F (10°C). Ventilate as needed during times of cool temperatures, to maintain the cellar in the ideal range. We need to actively manage conditions in our root cellar to cure the potatoes and help them store well. We have no automated ventilation, or even ventilation ducts. We simply leave the door open at night when we want to cool it down, or in the daytime in winter. We just choose a time when the forecast temperature is in the range we’re aiming for. Yes, mice do come in the open door! We encourage black snakes to live in our cellar, to keep the mice under control. (How do we encourage snakes? I mean we don’t drive them out, and if we need to, we move one or two in there.) This can be a bit unnerving, as the cellar is dark. (We chose not to have a light, as leaving it on by accident could cause a lot of potato greening before we noticed our mistake.) We have developed a special door-opening technique so we can co-exist with the snakes, who like to hang out on the top of the doorframe. We unlatch the door, open it a crack, then bang it closed, before opening it fully. Any resting snakes have by then dropped to the floor where we can see them and avoid them. (No snakes have been hurt in this process!) People who don’t like snakes will be really motivated to fit a rodent-proof vent system!
I wrote a blog post about Root cellar potato storage, on 8/07/2018. It includes a fuller version of our Root Cellar Warden instructions below. Here is the shorter, post-harvest version of our “Root Cellar Warden” instructions:
- After the potato harvest, the potatoes need to be at 60-75F (15-24C) with good ventilation for two weeks. Leave the door open on mild nights (or days) every 2 or 3 days, and close it later. The newly harvested potato is still respiring and needs fresh air. Lack of sufficient oxygen during curing results in Black Heart, a condition where the tubers develop nasty black lumps of dead tissue in the centers, so be sure to provide good ventilation during curing.
- After 14 days, the potatoes need sorting to remove Use First and Compost ones. Usually this is done by bringing the crates outdoors. You will need buckets, rags, gloves. It’s important to do this in the 3rd week after harvest, and not leave it longer, to minimize the spread of rot. Keep the crates away from walls, which sometimes collect condensation. The potatoes benefit from the airflow if they are not touching the walls.
- After 14 days, cool the cellar whenever a mild night or chilly day is forecast, down to 40-45F (4.5-7C).
Dormancy Requirements of Potatoes
We researched the dormancy requirements of potatoes in an effort to store ours so they don’t sprout when we don’t want them to.
What I know so far about dormancy is that potatoes need a dormancy period of 4-8 weeks after harvest before they will sprout. So if you plan to dig up an early crop and immediately replant some of the potatoes for a later crop, take this into account. Get around this problem by refrigerating them for 16 days, then chitting them in the light for 2 weeks. The company of apples, bananas or onions will help them sprout by emitting ethylene.
To avoid sprouting, keep the potatoes below 50F (10C) once they are more than a month from harvest, avoid excess moisture, and avoid “physiological aging” of the potatoes, caused by stressing them with fluctuating temperatures, among other things. If eating potatoes do start to develop sprouts, it’s a good idea to rub off the sprouts as soon as possible, because the sprouting process affects the flavor, making them sweet in the same way that low temperatures do.
I have also written blog posts about
How to Deal with Green Potatoes (one of my most-read blog posts!)
and if you still want to read more about potatoes,
Mary Jo Frazier and colleagues at the University of Idaho Extension, in 2004, researched the use of essential oils of mint and cloves to inhibit sprouting in storage. These plant oils can add 20-30 days storage, and then need to be reapplied. There is the issue of flavors carrying over into the tubers.
Other biocontrols to reduce storage losses
There has been some USDA ARS (Agricultural Research Service) research into biological disease control for stored fruit and vegetables. It takes three directions:
- Using biologicals such as Aspire yeast, Bio-Save Bacteria (Pseudomonas syringae) or chitins to form a semi-permeable film over the surface of the roots and fruits;
- UV light to induce rot resistance. Primarily used for fruit;
- Natural fungicides derived from jasmine and peaches, which induce disease resistance in the crop itself.
Currently these methods are only used by large operations, but in the future, they may be useful to small growers.